The present invention relates to a gunning refractory composition for use in lining troughs of blast furnaces or vessels for receiving molten pig iron, and more particularly it relates to a refractory composition for use in gunning having high adhesion ability, high strength, low porosity, and high corrosion resistance.
Heretofore, the lining of the troughs of blast furnaces and vessels for molten pig iron was formed using refractory bricks or an monolithic refractory applied by stamping, casting, vibration molding and the like. Recently, however, casting has become the predominant technique for the lining of troughs of blast furnaces in order to reduce labor and costs and to improve working conditions.
A casting material is composed of a castable refractory composition containing 15 to 25% by weight of conventional alumina cement. This castable refractory composition has drawbacks such as rupturing on rapid drying as the result of rapid dehydration of the alumina cement, poor strength in an intermediate temperature region, the reduction of resistance to slag, and the reduction of hot strength due to the presence of CaO in the alumina cement. These drawbacks, however, are substantially alleviated by employing a low cement castable refractory or an alumina cement free castable refractory in which superfine refractory powders and a deflocculant are used, while the amount of alumina cement to be used is made as small as possible, and the above-mentioned castable refractories, hereinafter referred to as low cement castable refractories, can be used without any problems from the standpoint of durability.
However, casting requires blending on-site of component materials by a large mixer and framing operations which are quite troublesome. It has other drawbacks such as a long curing period at the end of which the frame disengages and also a long drying period. Thus, casting is not yet a satisfactory application technique.
Gunning is preferable to casting as a technique for applying linings to troughs and vessels for molten pig iron, because it does not require on-site blending and framing. Thus, it permits reductions in labor and equipment costs and can be effectively used in repairing local damage to lining by cold or hot gunning.
In the case of conventional gunning operation effected by blending water and a powdery gunning material at the nozzle of a spray gun, the powdery gunning material being transferred by pressure to the nozzle, the water content is rather high (10-20% by weight of the total weight of blended water and powdery material.) This produces a lower adhesion ratio and a lower packed density of the adhered material than for a cast material containing 4-8% by weight of water, resulting in lower durability. For this reason, at present, gunning is used only for repairing local damage to linings. Therefore, many efforts have been made in order to reduce the water content of gunning refractory compositions, and extensive investigations have been made concerning the grain size of the particles in the particulate component, the selection of an effective binder for gunning refractory compositions, the nozzle shape of the spray gun, and the like. However, in spite of these efforts and investigations, a satisfactory gunning refractory composition has yet to be achieved. For example, the mere addition of a deflocculant to the gunning refractory composition is insufficient, since it require at least 3- 5 seconds to deflocculate superfine refractory powders. This is too long, for it takes less than 1 second for the gunning refractory composition to reach the target surface to be coated starting from the point when water is mixed with a powdery composition, and thus sufficient deflocculation cannot be achieved.
The present inventors have discovered that a low cement castable refractory composition containing a very low alumina cement, superfine powders and a deflocculant in combination and having a low water content can be successfully used as a gunning composition in order to obtain a lining of high packed density if a fine powder portion comprising a portion of fine powders of a refractory aggregate separated from a coarse/fine grain portions of the refractory aggregate, superfine refractory powders and a deflocculant are together slurried and if the resulting slurry is used in place of water of the conventional spray coating composition. Thus the slurry is mixed at a nozzle of spray gun with the remaining coarse/fine grains of refractory aggregate.